1. Field of the Invention
The present invention relates to a clock switching device which switches a clock to be supplied to an information processing device (computer device) and, more particularly, to a clock switching device capable of detecting abnormality in a waveform to switch a clock, and a clock switching method.
2. Description of the Related Art
When a failure occurs in a clock distribution system, because it is a conventionally practice to restart a device after replacing a part suspected to develop the fault, processing of the device should be interrupted at the time of recovering from the failure.
One example of conventional clock switching devices is recited in Literature 1 (Japanese Translation of PCT International Application No. H09-510338). While the clock switching device recited in Literature 1 proposes a circuit which matches phases of two input clocks to switch a clock, because it is premised on a clock of the same oscillation source (synchronization), when an oscillation source develops a fault, the processing of the device should be interrupted to replace a part suspected to develop the fault, so that it is necessary to interrupt the processing of the device to restart the device.
In addition, since the clock switching device has no function of detecting abnormality in a waveform, some other means should be provided for detecting a failure of a clock distribution system to instruct on switching of an input clock.
One example of clock switching devices which solves such a conventional problem is recited in Literature 2 (U.S. Pat. No. 5,903,748) as an asynchronous clock switching circuit with which no clock pulse is interrupted at the time of recovering from a failure of a device.
The above-described conventional clock switching device, however, has a problem of having a larger jitter than that of a clock generated by a quartz oscillator with high precision because of use of a VCO (voltage controlled oscillator).